DOCX filter: effect extent should be part of the margin

OOXML has an object size (extent) a size including effects like shadow (effect
extent) and the margin (top/bottom/left/right distance). Our doc model doesn't
have an explicit "effect extent", but to have the intended layout, we at least
need the spacing of the effect around flys (even if a number of effects are not
rendered). Fix this by adding the effect extent to the margin; if we still have
the effect extent grab-bag around, then this can be undone during export.

Change-Id: I9609ce347c54588395e09ad91a84c872b9007f85